Lawesson's Reagent is a mild and convenient thionating agent for ketones, esters, and amides that allows the preparation of thioketones, thioesters and thioamides in good yields.
Reactions using the comparable reagent P4S10 normally need higher temperatures and a large excess of the thionating agent.
Lawesson's Reagent in solution is in equilibrium with a more reactive dithiophosphine ylide:
The reaction with a carbonyl gives rise to a thiaoxaphosphetane intermediate:
The driving force is the formation of a stable P=O bond in a cycloreversion step that resembles a portion of the mechanism known for the Wittig Reaction:
Reactions of ketones, amides, lactams and lactones are normally faster than reactions of esters. Esters are unreactive depending on the reaction conditions, which allows selective transformations:
More detailed information can be found in a recent review by Jesberger, Davis, Barner (Synthesis, 2003, 1929. DOI)
Synthesis of 2-Alkoxy-Substituted Thiophenes, 1,3-Thiazoles, and Related S-Heterocycles via Lawesson's Reagent-Mediated Cyclization under Microwave Irradiation: Applications for Liquid Crystal Synthesis
A. A. Kiryanov, P. Sampson, A. J. Seed, J. Org. Chem., 2001, 66, 7925-7929.
Solution versus Fluorous versus Solid-Phase Synthesis of 2,5-Disubstituted 1,3-Azoles. Preliminary Antibacterial Activity Studies
J. F. Sanz-Cervera, R. Blasco, J. Piera, M. Cynamon, I. Ibáñez, M. Murguía, S. Fustero, J. Org. Chem., 2009, 74, 8988-8996.
Lawesson's Reagent and Microwaves: A New Efficient Access to Benzoxazoles and Benzothiazoles from Carboxylic Acids under Solvent-Free Conditions
J. A. Seijas, M. P. Vázquez-Tato, M. R. Carballido-Reboredo, J. Crecente-Campo, L. Romar-López, Synlett, 2007, 313-316.